The brightness of the optoelectronic device like LED is improved continuously in recent years, and the application field has been expanded from traditional indicator or ornament to the light source of all kinds of device. It is expected that in the coming future LED can replace traditional fluorescent light and be a light source of the next generation.
As shown in FIG. 1, the conventional LED structure has a single p-n junction. The basic structure comprises a substrate 13, an n-type semiconductor layer 11 on the substrate 13, a p-type semiconductor layer 12 on the n-type semiconductor layer 11, and a light-emitting layer 10 between the p-type semiconductor layer 12 and the n-type semiconductor layer 11.
As shown in FIG. 2, in order to improve the light extraction of unit area of an LED, the LED structure with multiple light-emitting stacks uses a tunnel layer 17 to connect the first p-n junction structure I and the second p-n junction structure II so the light extraction of unit area of the LED can be improved and the driving voltage of the LED is double. However, the driving current of the LED is not increased. The characteristic of high driving voltage and low driving current of LED is advantageous to lighting products. The tunnel layer 17 comprises a highly doped n+-type semiconductor layer and a highly doped p+-type semiconductor layer. Since the highly doped n+-type and the highly doped p+-type semiconductor layers have low light transmittance, the tunnel layer 17 should be thin for improving the light transmittance. However, if the tunnel layer 17 is too thin, dopants diffused from other semiconductor layers dope into the tunnel layer 17 easily and affect the function of the tunnel layer.